In microelectronics, high-temperature-resistant polymers are needed especially as insulation layers and dielectrics. Precursors of such polymers, such as polyimides (PI) and polybenzoxazoles (PBO) can be made photoreactive with suitable additives and can directly be structured photochemically. The precursors, which are also known as prepolymers, are converted to the corresponding polymer (PI or PBO) by a temperature treatment, thus yielding the final properties.
In contrast to most photosensitive PI precursors, photosensitive PBO precursors offer the advantage of structurability in a positive mode and the possibility of developing with an aqueous alkaline medium (see European Patent No. 23,662 and Unexamined European Patent Application No. 264,678). An important advantage of the positive structurability is the low defect density in structuring so-called "via holes," because only a fraction of the area is exposed--in comparison with systems that work negatively. In addition to having an adjusted alkali solubility, i.e., solubility in aqueous alkaline metal-ion-free developers, PBO precursors suitable for direct structuring must have a good solubility in organic solvents so that they can be applied to a substrate from a solution--by means of centrifugal or spray methods.
The most common method of synthesizing alkali-soluble PBO precursors is to react a dicarboxylic acid chloride with a suitable bis-o-aminophenol. As a rule a soluble base such as pyridine or triethylamine is added to capture the hydrogen chloride (HCl) formed in the reaction (see Unexamined European Patent Application No. 264,678 and European Patent No. 291,779). The residual chloride remaining in the PBO precursor and optionally other ions must be removed by means of ion exchangers because they can impair the functioning of.microelectronic components in the long run. However, this is complicated and expensive.
A requirement that must be met in synthesis of PBO precursors is that the dicarboxylic acid or dicarboxylic acid derivative must react mainly with the amino groups of the bis-o-aminophenol (forming an amide), but there must be little or no reaction with its hydroxyl groups (forming an ester), i.e., the reaction selectivity in formation of the amide in comparison with formation of the ester must be high. If the selectivity of the reaction is not high enough, the reaction will lead to polymers with inadequate alkali solubility or to gelation, i.e., then the PBO precursor cannot be used.
Methods of chloride-free synthesis of polybenzoxazole precursors-- and likewise polybenzothiazole precursors-- have already been described. Thus, it is known from Unexamined European Patent Application No. 158,726 that dihydroxy- and dimercaptodiamino compounds can be reacted with a dicarboxylic acid in the presence of a carbodiimide. However, urea residues remaining in the resin due to side reactions often cause problems in this reaction. They have a negative effect on the thermal stability of the polybenzoxazole or polybenzothiazole as well as the quality of layers produced from them. In addition, the polymers synthesized by this method do not have adequate solubility in aqueous alkaline metal-ion-free developers.
An alternative chloride-free synthesis process for poly-o-hydroxyamides consists of using condensation reagents such as 1-ethoxycarbonyl-2-ethoxy-1, 2-dihydrocuinoline or dicyclohexyl carbodiimide and 1-hydroxy-1,2,3-benzotriazole to react the dicarboxylic acid with bis-o-aminophenol (see Unexamined European Patent Application No. 391,196). However, the polymers synthesized by this method also have an inadequate solubility in aqueous alkaline metal-ion-free developers.
Another method of synthesizing polybenzoxazole and polybenzothiazole precursors in a chloride-free process is proposed in Unexamined European Patent Application No. 761,718, where bis-o-aminophenols or bis-o-aminothiophenols are reacted with dicarboxylic acid derivatives in the form of (thio)esters or amides with compounds based on benzoxazole, benzothiazole and benzotriazole, i.e., with so-called activated acid derivatives, such as activated esters. Polymers synthesized in this way are soluble in aqueous alkaline metal-ion-free developers, although the dicarboxylic acid derivatives used in this process must be synthesdized seately or isolated (see Unexamined European Patent Application No. 761,641). However, this is more expensive in comparison with direct synthesis, i.e., reacting a dicarboxylic acid with a bis-o-aminophenol, for example.
Unexamined German Patent Application No. 3,716,629 discloses thermally stable positive resists based on polybenzbxazole precursors (in the form of hydroxypolyamides) and diazoquinones. In addition to the actual starting materials, triphenylphosphine and hexachloroethane are used to synthesize the PBO precursors. Thus, the synthesis is not chloride free. In addition, the resists are developed with developers containing metal-ions.